问题
选择题
The research is so designed that once _____ nothing can be done to change it.
A．begins
B．having begun
C．beginning
D．begun
答案
答案：D
可视为once it is begun之省略。
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The research is so designed that once _____ nothing can be done to change it.
A．begins
B．having begun
C．beginning
D．begun
答案：D
可视为once it is begun之省略。